1. Field of the Invention
The present invention relates to a communication system, and more particularly, to a method of detecting a received signal in a communication system.
2. Discussion of the Related Art
As a method for a receiving side to receive a signal transmitted by a transmitting side, there is energy detection for determining a transmitted signal by detecting energy of the transmitted signal. The energy detection is used in detecting a signal transmitted by OOK (on-off keying) modulation scheme that is used in transceiving such simple information as ‘presence’ and ‘non-presence’. The OOK is the modulation scheme for representing digital data via presence or non-presence of a carrier. In a simple manner, it is able to represent a presence of a carrier for a predetermined duration as binary information ‘1’. And, it is also able to represent a non-presence of a carrier for the predetermined duration as binary information ‘0’.
For instance, the OOK modulation scheme is the scheme evolving from 3GPP WCDMA that is the asynchronous IMT 2000 mobile communication system and is used for HICH (hybrid ARQ indicator channel) and RGCH (relative grant channel) in 3GPP Rel-6 and 3GPP Rel-7 for implementing a system for bidirectional high speed packet transmission. And, the OOK scheme is used for E-HICH (E-DCH hybrid ARQ indicator channel) that is a channel for carrying ACK/NACK (acknowledgement/non-acknowledgement) information of uplink E-DCH (enhanced dedicated channel). According to this scheme, if information carried on E-HICH is ACK, ‘1’ is transmitted. If the information is NACK or it is unable to determine ACK/NACK due to DTX (discontinuous transmit), ‘0’ is transmitted. Besides, E-RGCH shall be described later in this disclosure.
A transmitted signal, which has been modulated by the OOK modulation scheme, is detected according to amplitude detection using high/low level of energy of the signal received by a receiving side. In this case, the amplitude detection is the scheme for raising detection probability of a received signal by lowering false alarm probability and miss probability of a detected signal. The ‘false’ means that a receiving side determines NACK/DTX information of a transmitting side as ACK information. And, the ‘miss’ means that a receiving side determines ACK information of a transmitting side as NACK/DTX information. In this case, the false of the detected signal can be more fatal than the miss thereof. In case of the miss detection of HICH, the missed signal is retransmitted by a transmitting side, whereby such a problem as additional consumption of radio resources, service delay and the like is generated. In case of the false detection of HICH, although retransmission of a corresponding signal is necessary, a transmitting side determines that the signal is correctly transmitted to a receiving side. Therefore, a fatal situation may take place. For example, reception of necessary data may not be achieved.
Generally, when a transmitting side transmits ACK, a probability density function (PDF) of a received signal can be represented as Raleigh distribution that differs in a distribution chart according to channel configurations of transmitting and receiving sides due to a migration of a transmitting side or an existence of multi-path. When a transmitting side transmits NACK or DTX, a probability density function (PDF) of a received signal can be represented as Gaussian distribution that differs in a distribution chart.
When a received signal is determined as NACK or ACK using a probability density function of a received signal, it is able to define a value of amplitude or power corresponding to a boundary of NACK/ACK as a prescribed threshold value. In this case, the threshold value is related to an allowable false alarm probability specified in a false alarm probability curve based on DTX in a signal transmitted by a transmitting side only. In particular, even if a probability density function of a received signal has the same distribution chart, a threshold value may be different to what value is designated to an allowable false alarm probability. In case of attempting to lower a false alarm probability, a threshold value can be raised. In case of attempting to raise a false alarm probability, a threshold value can be lowered. In the following description, a method for a receiving side to detect a signal carried over a fading or non-fading channel according to a related art is described.
Generally, a signal received by a receiving side can be represented in a manner of multiplying a transmitted signal by a channel component and then adding a noise thereto.y=√{square root over (Px)}xh+n  [Formula 1]
In Formula 1, ‘y’ indicates a received signal in a receiving side, ‘x’ indicates an OOK (on-off keying) modulated transmitted signal, and ‘h’ indicates a channel component. And, ‘Px’ indicates a power of the transmitted signal. Moreover, ‘n’ indicates a noise component.
The received signal y passes through a channel estimation and is then channel-compensated, as shown in Formula 2. In Formula 2, ‘h*’ indicates a conjugate complex number of the channel component and ‘y·h*’ indicates a channel-compensated received signal.y·h*=√{square root over (Px)}·x·|h|2+n·h*  [Formula 2]
In the above channel compensation, a receiving side multiplies a received signal by information of a channel estimated from such a reference channel as a pilot channel. Since channel information generally includes size information together with phase information, channel compensation is performed in a manner of multiplying the received signal by a conjugate complex number of channel information obtained for compensation for size distortion and phase distortion attributed to channel influence.
A receiving side should determine whether a transmitted signal is ‘0’ or ‘1’ for a channel-compensated received signal. In this case, in order to reduce a false alarm probability and a miss occurrence probability and to raise a detection probability, a normalization scheme is applied. In particular, a distribution of a received signal is modified suitably for an energy detection scheme through a normalization process.
Two kinds of methods are generally provided for the normalization scheme. First, a received signal is normalized into a channel component. Second, a received signal is normalized into a noise component. The channel component normalization method can be categorized into a method of normalization into a power component of channel and a method of normalization into an amplitude power component of channel. In the following description, the channel compensation and/or normalization scheme shall be named a pre-processing.
Formulas 3 to 5 show representative schemes for normalizing a channel-compensated received signal, in which normalizations are executed using channel power component, channel amplitude component and noise signal power, respectively. In general, a noise signal is represented as Gaussian distribution (Size 0: center, Dispersion: σ2). In this case, a power of the noise signal means a mean power of the noise signal. And, the mean power of the noise signal is σ2.y·h*/|h|2=(√{square root over (Px)}·x|h|2+n·h*)/|h|2  [Formula 3]y·h*/|h|=(√{square root over (Px)}·x|h|2+n·h*)/|h|  [Formula 4]y·h*/σ2=(√{square root over (Px)}·x|h|2+n·h*)/σ2  [Formula 5]
In a related art system, one of the schemes shown in Formulas 3 to 5 is selected according to a system and is then fixedly used for the corresponding system.